Identification and Distribution of Metal Soaps and Oxalates in Oil and Tempera Paint Layers in Fifteenth-Century Altarpieces Using Synchrotron Radiation Techniques

The formation and distribution of metal soaps produced as a result of the reactivity and aging of the materials in a fifteenth-century egg tempera and oil paintings on wood are presented. The painting technique involves the application of several paint layers over a ground using, sometimes in the same paint layer sequence, drying oil and egg yolk binders. We show, with a selection of examples, how the use of thin sections and a combination of various micro-sensitive analytical techniques is adequate to obtain the high-quality data necessary for the unambiguous identification of metal soaps and metal oxalates as well as their distribution in the paint layers. The techniques include micro infrared spectroscopy (μSR-FTIR) and micro X-ray diffraction (μSR-XRD) with synchrotron radiation, optical microscopy (OM), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). The data obtained sheds light about the underlying reaction and aging mechanisms happening in each paint layer and among them. This helps to define the state of conservation of the artworks.